Rotary positive displacement pumps, such as vane pumps or internal-external gear pumps are well known and are widely used in a variety of environments. As used herein, the term rotary positive displacement pump is intended to comprise vane pumps, gear and crescent pumps, internal-external gear pumps, etc. Further, internal-external gear pumps include pumps which have a rotor set that includes an outer rotor having a given number of lobes and an inner rotor with at least one less lobe. The inner rotor is driven and rotates within and with the outer rotor and the lobes of the inner rotor moving into and out of the lobes of the outer rotor form a series of pump chambers. Examples of internal-external gear pumps include gerotor pumps, trocoid pumps, duocentric pumps, hypocycloid pumps etc.
One common use for rotary positive displacement pumps is to supply and pressurize a working fluid, such as lubrication oil, for a prime mover device. For example, internal-external gear pumps are typically used to supply lubrication oil to internal combustion engines and the like.
In such uses, the oil pump is typically driven by the internal combustion engine and thus the operating speed of the oil pump changes with the engine operating speed and, as the operating speed of the pump changes, the output volume of the oil pump changes. The lubrication system of the engine can be viewed as a fixed size orifice, and thus changes in the output volume of the pump result in changes in the output pressure of the lubrication oil.
While the output pressure of the oil pump varies with the engine operating speed, at the same time, the lubrication oil pressure requirements of the internal combustion engine also vary with the operating speed of engine. However, the lubrication oil pressure requirements vary with the engine operating speed in a different manner than the output pressure of the oil pump varies with the operating speed of the engine and thus either a variable displacement pump, and suitable control, must be employed or a fixed displacement pump with a control mechanism is required to alter the output pressure of the oil pump to avoid undesired and/or unsafe operating conditions.
With fixed displacement pumps, the control mechanism employed is typically a form of pressure relief valve, where the valve has a spring biasing it to a first position wherein the full output of the pump is available to the engine. A control chamber is supplied with pressurized oil from the pump and this pressurized oil in the control chamber creates a force on the valve to move it against the biasing spring, from the first position, to a second position where some portion of the output of the pump is returned to a low pressure sink, such as the oil sump or the pump inlet.
Because the volumetric displacement of the oil pump must be sufficient to meet the engine lubrication requirements at relatively low pump operating speeds, generally the output of the oil pump is too high at higher operating speeds and the pressure relief valve allows some of the output of the pump to be returned to the low pressure sink.
While such fixed displacement pump systems are widely employed, they do suffer from some disadvantages. In particular, due to limitations in the operation of the pressure relief valve, the output of the pump exceeds the output required by the engine at many points of those expected operating conditions. When the pump output exceeds the engine operating requirements, engine energy is being wasted pressurizing lubrication oil which is not needed by the engine.
When a variable displacement pump is employed, such as a variable displacement vane pump, the pump includes a member or mechanism which is moved to alter the volumetric displacement of the pump as need. The control mechanism acts to move the member or mechanism as needed to alter the output of the pump. A variety of control mechanisms are known for variable displacement pumps, including: control pistons which are supplied with pressurized working fluid from the output of the pump and which act against a biasing spring; and single chamber or multi-chamber systems wherein pressurized working fluid from the output of the pump act directly on the control member or mechanism against the biasing force of a spring.
While variable displacement pumps tend to provide more energy efficient results, they also suffer from similar problems to those of fixed displacement pumps in meeting, but not exceeding, the pressure requirements of a prime mover which change with the operating speed of the prime mover and pump.
It is desired to have a rotary positive displacement pump with a control mechanism that provides for the output of the pump to more closely match the requirements of a prime mover device supplied with a working fluid, such as an internal combustion engine, at a reasonable cost.